This invention relates to thin film battery construction, and more particularly to a method of producing the components of a thin film battery.
Lithium-ion batteries have existed for many years. These batteries have high energy and power densities as well as the capability of being cycled thousands of times. The active battery components are a lithium intercalation cathode, e.g. V2O5, LiMn2O4 or LiCoO2, and a lithium metal anode separated by an electrolyte. It has been found the LiCoO2 cathodes have the highest energy and power densities making them attractive as compact rechargeable power sources for application in a varies of electronic devices.
However, thin film lithium batteries cannot be integrated into electronic circuits using the solder reflow method as the temperature required for this process, 250xc2x0 C.-290xc2x0 C., is well above the melting point of lithium metal, 180xc2x0 C. Recently thin film lithium-ion batteries have been developed that are based on inorganic oxynitride and nitride anodes such as silicon-tin oxynitride, tin nitride, or zinc nitride. These batteries can withstand solder reflow temperatures with no adverse effect on battery performance.
However, upon the initial charging of the lithiumion battery approximately one half of the lithium from the LiCoO2 cathode is irreversibly lost to the anode because the oxygen and/or nitrogen in the anode reacts with the lithium to form Li2O and/or Li3N accompanied by the precipitation of silicon and/or tin. Subsequently cycling of the battery occurs through the remaining lithium found in the LixSi and/or LixSi alloys formed in the anode. For example, the reaction on initial charge of a cell with a tin nitride (Sn3N4) anode is:
252 LiCoO2+5Sn3N4=3 Li22Sn5+20 Li3N+252 Li0.5CoO2
The anode consists of a lithium-tin alloy (Li22Sn5) in this instance dispersed throughout a solid matrix of Li3N. After the initial charge, the following discharge-charge cycles occurs through the exchange of lithium between the Li22Sn5 in the anode and the LiCoO2 in the cathode:
252 Li0.5CoO2+3 Li22Sn5=252 Li0.76O2+15 Sn
Accordingly, it is seen that a need remains for a method of producing a thin film battery with minimal capacity loss on the initial charge. It is to the provision of such therefore that the present invention is primarily directed.
In a preferred form of the invention a method of producing a battery cell anode comprises the steps of providing a LixSny target and depositing the LixSny target in an argon-nitrogen atmosphere to form a Sn:Li3N anode, the nitrogen being limited to between 0.5% and 15% of the total volume of the argon-nitrogen atmosphere.